Start 'Std' module, move Common over

[WardBrian]

The first PR in the stacked version of #1655

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.31%. Comparing base (b96c001) to head (48aa147).

Files with missing lines	Patch %	Lines
src/frontend/Semantic_error.ml	75.00%	1 Missing ⚠️
src/std/Nonempty_list.ml	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1656      +/-   ##
==========================================
- Coverage   92.31%   92.31%   -0.01%     
==========================================
  Files          67       68       +1     
  Lines        9972     9975       +3     
==========================================
+ Hits         9206     9208       +2     
- Misses        766      767       +1     

Files with missing lines	Coverage Δ
src/common/Files.ml	100.00% <ø> (ø)
src/frontend/Typechecker.ml	94.69% <100.00%> (ø)
src/std/std.ml	100.00% <100.00%> (ø)
src/frontend/Semantic_error.ml	96.21% <75.00%> (ø)
src/std/Nonempty_list.ml	75.00% <0.00%> (ø)

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[WardBrian]

Thanks @nhuurre - I'm not sure entirely how merging will affect the stack or the unstacked version in #1655, so I'm gonna hold off for a bit until the other pieces can get reviewed